This invention relates to an error-compensated pressure measuring apparatus, and more particularly to an apparatus increasing the accuracy of a strain-gauge pressure transducer by digitally compensating for temperature-induced and device-specific errors.
A conventional silicon strain gauge pressure transducer includes a thin silicon diaphragm with four implanted resistors. Pressure variations on the diaphragm strain the resistors causing their respective resistance values to change. The resistors are interconnected in a bridge configuration having input and output connections. When pressure is applied to the diaphragm, the bridge structure exhibits compression at a first two resistors and tension at the second two resistors. The compressed resistors have a decreased resistance, while the tensed resistors have an increased resistance. Variations in resistance cause variations in bridge signal characteristics.
In operation, an excitation current is applied to the bridge (diaphragm) via two input connectors. An output signal is measured at two output connectors. The output signal typically is in the millivolt range. Changes in resistance caused by changes in pressure vary the output signal. In an ideal pressure transducer, the individual resistances would change only in response to variations in pressure. However, temperature variations also alter the resistance values. Other sources of error also may effect the resistance values. In the past, one solution has been to avoid or readily compensate for temperature induced error by limiting a strain gauge's operational temperature range to a narrow field. Many applications, however, require operation over a wide temperature range.
Passive compensation schemes provide one solution for compensating for temperature in a strain gauge over a wide temperature range. According to alternate passive compensation schemes, either a constant voltage or a constant current excitation signal is applied to the bridge. For a constant current excitation signal, sensors are supplied with a compensation card including several thermally stable resistors. Resistance values are determined from test data. For a constant voltage excitation signal, a thermally stable, precision resistor is coupled in series to the bridge. Actual excitation current is then derived by measuring the voltage drop across the precision resistor. The passive compensation schemes partially corrects for temperature-induced offset errors. Using these passive compensation schemes reduces error bands to as low as 1% of full scale pressure (i.e., 1% FSO) over a 0.degree. C. to 50.degree. C. range. However, such accuracy still is unacceptable for many pressure measurement applications.
For applications requiring error bands less than 1% FSO, other sensor technologies have been used. For example, mercury manometers provide pressure reading error bands down to 0.1% FSO over an operating temperature range of 0.degree. C. to 50.degree. C. Different applications require different accuracies. One typical application of a pressure sensor is measurement of water depth. When measuring water depth in a well, the temperature is generally stable (e.g., varies by less than +/- 5.degree. C. over the course of a year). The less accurate, less expensive conventional strain gauge is sufficiently accurate for such measurement. For example, the strain gauge can be calibrated with a zero offset to get sufficient measurement accuracies. When measuring water depth in a stream, river, lake or ocean body, however, the temperature usually varies significantly with changes in depth and over the course of a year for the interested depth range. As a result, the typical strain gauge cannot meet accuracy requirements. In such applications, the mercury manometer has been commonly used. Mercury in the waterways, however, has led to regulations limiting many sources of mercury contamination. These regulations are limiting the use of mercury manometers. Accordingly, there is a need for an environmentally safe technology which can provide accurate pressure measurements over a wide operating temperature range.